Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
In a typical cellular wireless communication system, the air interface between a base station and served user equipment devices (UEs) may define a downlink (or “forward link”) for carrying communications from the base station to the UEs and an uplink (or “reverse link”) for carrying communications from the UEs to the base station. In various implementations, these links may be defined on one or more carrier frequencies or blocks of frequencies. Furthermore, on each link, various channels may be defined through techniques such as time division multiplexing, code division multiplexing, and/or frequency division multiplexing.
In one implementation, the downlink and uplink may each be divided over time into a continuum of timeslots for carrying communications between the base station and the UEs. UEs may be arranged to transmit data, such as bearer traffic (e.g., user data) and control signaling (e.g., access requests and the like), to the base station in such timeslots on the uplink, and the base station may be arranged to transmit data, such as bearer data and control signaling (e.g., paging and other overhead messages) to the UEs in such timeslots on the downlink. Further the base station or associated equipment may dynamically schedule transmissions of data in particular timeslots, and the base station and UEs may transmit according to that schedule.
In practice, a UE may, from time to time, determine its channel quality based on downlink air interface quality (e.g., downlink reference signal strength) and one or more other factors (e.g., the UE's specific antenna structure and receiver capability) and transmit to the base station a channel quality indicator (CQI) indicating the UE's determined channel quality. The base station may then determine from the reported CQI a particular modulation and coding scheme (MCS) to use for communication with the UE, and the base station may allocate air interface resources and schedule transmissions of data accordingly.
Some air interface protocols support both wideband and sub-band CQI reporting. With wideband CQI reporting, a single CQI value may represent an effective signal-to-interference-plus-noise ratio (SINR), for example, as observed by the UE over an entire channel bandwidth. While a channel-wide CQI value may reveal an overall channel quality, a channel-wide CQI value also masks variations in SINR across the channel that may be present due to frequency selective fading (deviation of the attenuation affecting a signal which varies with frequency) and/or inter-cell interference (interference from an adjacent base station that is communicating with other UEs).
On the other hand, with sub-band CQI reporting, a UE may be arranged to evaluate the reference signal in particular sub-bands of the channel bandwidth, and to provide the base station with a sub-band CQI report specifying CQI values respectively for various sub-bands. Such sub-band CQI reports, which are of a finer frequency granularity than channel-wide CQI reports, allow the base station to engage in frequency selective scheduling (FSS), whereby a base station schedules transmissions of data with the goal of allocating to each UE the resources that the base station estimates will be received with the best SINR by the UE.
Furthermore, to aid the base station in scheduling and allocating resources for transmission of data on the uplink, the base station may be arranged to evaluate the uplink channel quality. In order to facilitate this evaluation, UEs may be configured to periodically transmit to the base station an uplink control signal, such as a sounding reference signal (SRS). The reception quality of the uplink control signal may then be used by the base station to help evaluate uplink channel quality. For instance, evaluating the reception quality of the uplink control signal in the frequency domain may allow the base station to engage in FSS of the uplink resources. By way of example, the base station may allocate for use by a UE the particular resources that the base station estimates being able to receive with the best reception quality from the UE.
Additionally, the base station may be configured to engage in carrier aggregation. In wireless communication systems that support carrier aggregation, multiple carrier frequencies from either contiguous frequency bands or non-contiguous frequency bands can be aggregated to increase the bandwidth available to a UE. By way of example, according to the Long Term Evolution (LTE) Advanced air interface protocol, the maximum bandwidth for a data transaction between a base station and a UE using a single carrier frequency is 20 MHz. A base station may increase the maximum available bandwidth by engaging in carrier aggregation. For instance, the base station may increase the number of resource blocks provided to a UE by aggregating up to five carrier frequencies, and consequently increasing the maximum bandwidth to up to 100 MHz. Each aggregated carrier frequency is referred to as a component carrier.